Episode 21 of "Sousou no Frieren" begins with a conversation between Frieren and Denken, in which Denken's philosophy emphasizes that the true essence of magic lies in the search for it rather than in its acquisition. This view resonates with Frieren and is later confirmed when Serie offers her any spell of her choice. Flamme, Serie's apprentice, introduces Frieren and sparks a discussion about the nature of magic and ambition. Flamme prophesies that Frieren will play a decisive role in defeating the Demon King and ushering in an era of peace, which stands in contrast to Serie's combative nature.

Meanwhile, Lawine and Kanne find themselves in a precarious situation against Richter, whose superior magic and tactical finesse put them at a significant disadvantage. Richter educates them about the evolution of magic and emphasizes the strategic importance of physical attacks in modern sorcery. Despite the unfavorable circumstances, Lawine skillfully uses her knowledge against Richter, exploiting his underestimation of her abilities.

At the same time, Denken engages in a strategic battle with Frieren, aiming to deplete her mana. However, Frieren's tactical prowess and deep understanding of magic allow her to turn the tide and demonstrate her superiority without relying on advanced spells.

The episode also reveals personal motivations and deeper connections of the characters to their magical journey. Denken's desire to become a first-class mage is driven by his wish to visit a grave in his homeland, which has been plagued by demonic activity.

The climax of the episode showcases Frieren masterfully manipulating the situation to her advantage. By breaking through the barrier that Serie had described as indestructible, she shifts the battlefield conditions in favor of Lawine and Kanne, allowing them to exploit Richter's vulnerability to water. This decisive move not only highlights Frieren's strategic genius but also her belief in the freedom and creative use of magic, paving the way for their escape.